Can gravitons travel in a vacuum?

The minute a spaceship is out of the earths atmosphere and into the vacuum, the gravitational pull lets go and the spaceship is free.
So what I'm asking is, how can the sun attract the earth into its pull?

Gravitational pull never "lets go". It drops off very rapidly (as 1/r2) but there is always some force (and the sun is very massive!). A lot of people confuse "free fall" with gravity "stopping". You feel gravity when there is something (like the earth or a floor) preventing you from going down. In space, when you shut off the rocket engines, you are in "free fall" because you are now "falling freely" with nothing to stop you. You could get the same effect by stepping off a tall building (except it won't last as long!).

gravitons are just hypothetical particles to link Quantum Mechanics, General Relativity and gravity. they carry the force of gravity in (dont hold me to this)
wave packets. in Newtonian physics as we are talking about in this thread, they really dont matter.